The invention has been especially developed in connection with heatable rollers for drying and thermosetting in a stretching machine for felt and wire cloths which are used in paper-making machines. The heatable roller is attached to a heat shield so as to obtain heat impact on the felt/wire cloth from the outside.
Oil-heated rollers, which today are the most commonly used heatable rollers in a stretching machine, are heated by thermo-oil flowing through air ducts or channels in the roller jacket. These ducts or channels are oriented in the cross-machine direction. The oil flow through the channels may be based upon a one-way flow or may be based upon the counterflow principle (alternating flow direction in adjacent channels). The oil acts as a thermofluid for the transport of heat to the roller. The oil temperature will fall en route as the oil emits heat, and this results in a temperature fall in the channels in the cross-machine direction. In a heating period, depending upon how rapidly the roller is heated, the temperature fall in the channels can be quite noticeable. However, once the roller has reached the chosen working temperature, the temperature fall in the oil will be low.
Hitherto, it has been usual to design the oil-heated heat shields according to the same principles as the rollers, with ducts or channels integrated into the shield and extending in the cross-machine direction.
A felt or wire cloth which passes over a heated roller will be subjected to expansion and contraction in the cross-machine direction. Along the free edges of the band, the band will expand or contract relatively freely, but in the interior of the band cross-section, that is in the centre of the band the band will have a poorer capacity for expansion. In order to achieve a more even felt quality, it is desirable to be able to control the temperature in the cross-machine direction, precisely because the felt in the centre is firmly fixed. Clearly therefore, there is a need to supply more or less heat along the edges of the band than in the center, so as to have control of and enhance the quality of the felt.